Power management

Think of power management as being akin to volume control on a stereo system.
Low volume is suitable for safe, controlled practice.
The higher the volume, the more unpredictable the outcome.
Martial arts are meant to be lethal, so full power means killing somebody and no one wants to do that.

Full contact or full power?

Full contact is commonly assumed to mean full power.
This is not accurate.
A full power karate punch would kill.
A full power judo, ju jitsu or aikido application would break a limb or concuss the attacker.